1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic latent image formed in an image forming method, such as an electrophotographic method, an electrostatic recording method or an electrostatic printing method, a toner used in an image forming method according to a toner jet recording method, and a process for producing a toner.
2. Description of the Related Art
As described, for example, in U.S. Pat. Nos. 2,297,691 and 2,357,809, an electrophotographic method is a method for forming an image by uniformly charging a photoconductive insulating layer, exposing the layer, forming an electrostatic latent image, developing the electrostatic latent image by a toner (a developing process), transferring the obtained toner image onto a transfer material, such as transfer paper, via an intermediate transfer member, or without using an intermediate transfer member (a transfer process), and fixing the toner image on the transfer material according to a heating fixing method, a pressing fixing method, or a heating/pressing fixing method (a fixing process).
Since image formation is performed through various processes as described above, the toner for forming a toner image must have functions required in the developing process, the transfer process, and the fixing process.
In general, since the toner receives a mechanical frictional force due to a shearing force and an impact force received during a mechanical operation within a developing device, it tends to be degraded during copying or printing operations for several to several tens of thousands of sheets. In order to prevent such degradation of the toner, a tough binding resin having a large molecular weight capable of resisting a mechanical frictional force may be used. In general, a binding resin having a large molecular weight has a high softening point. Hence, in order to sufficiently fix a toner image in a heat-roller fixing method which is widely being used because of a contact fixing method and excellent thermal efficiency, a heat roller must be set to a high temperature. A high temperature of the heat roller will cause degradation of the fixing device itself, the generation of curl of paper after fixing, and an increase in energy consumption. Furthermore, since such a binding resin also has poor grindability, the efficiency in manufacture of the toner decreases.
As described above, the heat-roller fixing method has superior thermal efficiency because the surface of a heating roller is in pressure contact with the toner-image surface of a transfer material, and therefore is widely used in a wide range of machines from low-speed machines to high-speed machines. In the heat-roller fixing method, however, when the surface of the heating roller contacts the toner-image surface, an offset phenomenon tends to occur in which the toner adheres to the surface of the heating roller and the adhering toner is transferred onto the succeeding transfer material.
In order to improve an offset resisting property of a toner, there have been proposed toners which use a covalent cross-linking resin or a branching resin using a multifunctional monomer, called a cross-linking polymer, or a multifunctional initiator (described in Japanese Patents Laid-Open Application (Kokai) Nos. 3-203746 (1991) and 4-24648 (1992)), and toners which use an ionic cross-linking polymer obtained by strongly bonding a metal oxide and a polymer (described in Japanese Patents Laid-Open Application (Kokai) Nos. 61-213858 (1986) and 6-175395 (1994)). Although the offset resisting property is improved in these toners, the fixability of the binding resin itself is degraded. Furthermore, since polymer molecules are strongly bonded to each other, resin components due to cross-linking represented by tetrahydrofuran-insoluble matter cause difficulty of dispersion of a coloring agent or a charge control agent into a binding resin, and degrade the grindability of a toner mixture during manufacture of a toner.
In general, since the minimum fixing temperature is between low-temperature offset and high-temperature offset, the usable temperature range is between the minimum fixing temperature and high-temperature offset. By reducing the minimum fixing temperature and increasing the temperature of generation of high-temperature offset as much as possible, it is possible to reduce the fixing temperature and to widen the usable temperature range. As a result, it is possible to realize energy saving and high-speed fixing, and prevent the occurrence of curl of a sheet. Since curl of a sheet can be suppressed, it is possible to perform copying on two surfaces, and realize an intelligent copier, precise temperature control of a fixing device, and mitigation of allowance.
For that purpose, a toner having fixability at a low-temperature and an excellent offset resisting property is desired.
Recently, in accordance with spread of use of computer apparatuses for personal users, full-color image communication has been widely utilized as an information transmission mechanism using images. Under such circumstances, full-color approach has also been rapidly adopted in printers and copiers, serving as output apparatuses, particularly in low-grade apparatuses, so that color images are becoming familiar to ordinary users.
In full-color approach, colors are reproduced using toners of three colors, i.e., yellow, magenta and cyan which are three primary colors, or toners of four colors including black in addition to the three colors. For example, an electrostatic latent image for magenta is formed on a photoconductive layer by passing light from an original through a color-separation-light transmitting filter in a relationship of a complementary color with magenta (a latent-image forming process). Then, a magenta toner image is held on a supporting member after passing through a developing process and a transfer process using a magenta toner. These processes are sequentially performed also for a cyan toner, a yellow toner and a black toner. The color toners are superposed on the same supporting member by being subjected to registration, and a final full-color image is obtained by fixing the superposed images.
Particularly in color toners, it is important that the fixed color image is in a state of substantially complete fusion such that the shapes of toner particles can be hardly discriminated, so as not to hinder color reproduction due to irregular reflection of light, and that a color toner is transparent so as not to hinder the color tone of a layer of a color toner having a different color tone present below the first-cited color toner, and that each color toner has a hue and a spectral reflectance in good balance and a sufficient saturation.
Many binding resins are being studied from the above-described viewpoint, and a color toner satisfying the above-described characteristics is expected. Nowadays, polyester-type resins are widely used in the concerned technical field as binding resins for color toners.
Japanese Patent Laid-Open Application (Kokai) No. 57-124740 (1982) describes that, in order to suppress reaction of free radicals during synthesis of a binding resin, it is desirable to add a hindered-phenol-type antioxidant by a total amount of 0.1-3.0 weight % with respect to a high-molecular polymer.
On the other hand, Japanese Patent Laid-Open Application (Kokai) No. 1-246560 (1989) describes that, in order to suppress the oxidation of both a toner using a binding resin, and a photosensitive member comprising an organic photoconductor, it is desirable to add a hindered-phenol-type antioxidant by a total amount of 0.01-10 weight % with respect to the toner during mixture of materials before manufacturing the toner by fusing and kneading the binding resin with other materials.
However, the effect of addition of such a hindered-phenol-type antioxidant is not realized in all toners and toner-manufacturing methods. When adding such an antioxidant to a toner having the configuration of the present invention, it is necessary to pay close attention to the amount of addition and the method of manufacturing the toner.